1. Field of the Invention
The invention in general relates to superconducting coils and more particularly to apparatus for maintaining the coil at the proper temperature for superconducting operation.
2. Description of Related Art
An electrical coil is capable of storing energy in the magnetic field produced by current flowing through the coil. If the coil is a superconducting solenoid or toroid, extremely large amounts of energy can be stored for relatively long periods of time due to the fact that once in the superconducting state, resistance of the coil winding approaches zero ohms enabling the winding to carry large currents with essentially zero loss.
Superconducting magnetic energy storage (SMES) systems find use in various fields such as industrial, transportation, and defense, as well as in the electrical utility industry. For example, SMES systems are being proposed for energy storage as part of Flexible AC Transmission Systems (FACTS) and custom power equipment for distribution-level power quality improvement.
SMES is an attractive option for these systems due to its relatively high energy storage density and available discharge rates at the multi-megawatt level. A bath-cooled SMES magnet fabricated using a low temperature superconductor (LTS) must be maintained near 4.2K by immersion in liquid helium or other suitable cryogenic fluid. If the SMES magnet is fabricated using a high temperature superconductor (HTS) it may be able to operate at a temperature somewhat above 4.2K, but will nevertheless require a cryogenic fluid for maintaining the magnet at its operating temperature.
Typically the magnet is a solenoid with a relatively large external magnetic field. The external magnetic field can be greatly reduced by using instead a toroidal coil configuration, but a toroid is more expensive to fabricate than a solenoid.
The bath-cooled solenoidal or toroidal SMES magnet is contained in a vessel called a cryostat. Cryostats are typically double-walled, vacuum-insulated vessels and are generally fabricated from stainless steel alloy, a relatively poor heat conductor. The cryogenic fluid (along with the SMES magnet) is contained in the inner vessel where it is thermally isolated (to a great extent) from the external environment. As will be described, such arrangement is extremely costly, not only when the SMES is in a standby condition but also when it is in service delivering power.
The present invention provides an arrangement which significantly reduces the operating costs with respect to the usage of cryogenic fluid.